Mg-gd-y-zn-zr alloy and process for preparing the same

ABSTRACT

A Mg—Gd—Y—Zn—Zr alloy with high strength and toughness, corrosion resistance and anti-flammability and a process for preparation thereof are disclosed. The components and the mass percentages thereof in the Mg—Gd—Y—Zn—Zr alloy are: 3.0%≤Gd≤9.0%, 1.0%≤Y≤6.0%, 0.5%≤Zn≤3.0%, 0.2%≤Zr≤1.5%, the balance being Mg and inevitable impurities. The process for preparation thereof comprises: adding pure Mg into a smelting furnace for heating, then introducing mixed gases of CO 2  and SF 6  into the furnace for protection; adding other raw materials in sequence when the pure Mg is completely melted; preparing an ingot; conducting a homogenization treatment on the ingot prior to extrusion; conducting an aging treatment on the extruded alloy. A wrought magnesium alloy having superior overall performances and good fracture toughness, corrosion resistance and anti-flammability, with a small amount of rare earth element is obtained by adjusting the proportion of the alloy elements and by conventional casting, extrusion and heat treatment processes. The cost of the alloy is reduced while the strength of the alloy is maintained.

TECHNICAL FIELD

The present invention belongs to the metal materials and metallurgicalfield, and specifically relates to a wrought Mg-RE alloy and a methodfor obtaining a magnesium alloy with excellent overall performances byadjusting the contents of alloy elements (Gd, Y and Zn) or modifying hotworking processes.

BACKGROUND OF ART

As the magnesium alloy has many advantages, such as low density, highspecific strength, high specific stiffness, excellent dampingperformance and good castability, a boom in the development andapplication of the magnesium alloy has been started in the world since1990s. The magnesium alloy has a wide prospect of application inaerospace, automobile, high-speed rail, and 3C fields. However, theabsolute strength of the magnesium alloy is low, and the plasticity,flame retardant property and corrosion resistance are poor, which limitsthe large-scale application of the magnesium alloy. Therefore, itbecomes particularly important to develop a magnesium alloy withexcellent overall performances.

The rare-earth containing magnesium alloys have excellentroom-temperature mechanical properties and heat resistance. Kawamura etal. prepared an ultra-high strength Mg₉₇Zn₁Y₂ alloy with theroom-temperature yield strength greater than 600 MPa by employing rapidsolidification and powder metallurgy technology, however, thecomplicated and dangerous preparing process greatly increases thepreparation difficulty and cost, which limits the wide application ofthe alloy. Homma et al. prepared a rare-earth containing magnesium alloywith excellent mechanical properties by employing conventional casting,extrusion and heat treatment processes, the tensile strength and yieldstrength thereof at room temperature are 542 MPa and 473 MPa,respectively, and the elongation thereof is 8%. However, the rare-earthcontent in this alloy reaches up to 16 wt. %, and it not only increasesthe material cost, but also increases the density of the alloy, whichweakens the advantage of the magnesium alloy as a light material. Jianet al. added 1.8 wt. % Ag into Mg—Gd—Y—Zr alloy prior to rollingdeformation, and the room-temperature tensile strength and yieldstrength of the alloy reached 600 MPa and 575 MPa, respectively,meanwhile, it has a elongation of 5.2%. However, the addition of Ag in ahigh content results in a significant increase in the material cost,while the corrosion resistance of the alloy is also decreased, which isnot beneficial for the practical application of the magnesium alloy. Inaddition, as compared with other common metal materials, the magnesiumalloys generally have a lower ignition point. Such a relative stronginflammability hinders the applications of magnesium alloys in manyfields, especially in aerospace field. It also requires more intensiveresearches in the corrosion resistance.

The patent application No. CN201110282459.1 discloses a magnesium alloywith high toughness and high yield strength. After homogenizationtreatment, extrusion and aging treatment, the tensile strength of thealloy can reach to 360 MPa, the yield strength can reach to 330 MPa, andthe elongation can reach to 11%. The patent application No.CN201110340198.4 discloses a high-strength and heat-resistant magnesiumalloy with low rare earth content and its preparation method”. Afterhomogenization treatment, extrusion and aging treatment, the alloyexhibits a tensile strength≥250 MPa, and an elongation≥8%. The patentapplication No. CN200510025251.6 discloses a high-strength andheat-resistant magnesium alloy and its preparation method. The magnesiumalloy in T5 state can exhibit a tensile strength≥369 MPa, a yieldstrength≥288 MPa, and an elongation≥5.1%. The mechanical properties ofthe rare-earth containing magnesium alloys involved in the above patentsare relatively low, and it is difficult to apply them in bearingcomponents in a large amount.

The patent application No. CN201210164316.5 discloses a high-strengthMg—Gd—Y—Zn—Mn alloy. After homogenization treatment, extrusion and heattreatment, the alloy can exhibit a tensile strength≥428 MPa, a yieldstrength≥241 MPa, and an elongation≥7.7%. The patent application No.CN201410519516.7 discloses preparation and treatment processes of aMg—Gd—Y—Zr alloy. After T5 treatment, the highest mechanical propertiesthereof are: a tensile strength of 403 MPa, a yield strength of 372 MPa,and an elongation of 4.4%. The patent application No. CN201610122639.6discloses a Mg—Gd—Y—Ni—Mn alloy with high strength and high plasticityand its preparation method. Its highest mechanical properties can reachto a tensile strength≥450 MPa, and an elongation≥9.0%, but therare-earth content of the alloys listed in this patent are about 12%,leading to a high cost. The rare-earth content of the alloys involved inthe above patents are all high, resulting in increasing cost and densityof the alloys, which is not beneficial for the widely industrialapplications.

The patent application No. CN201010130610.5 discloses a flame-retardantmagnesium alloy containing Gd, Er, Mn and Zr, wherein its flameretardant temperature can reach to 740° C., the room-temperature tensilestrength of the cast alloy can reach to 220 MPa, and the elongation islarger than 5%. The patent application No. CN201210167350.8 discloses aflame-retardant magnesium alloy, wherein the elements such as Ca, Sr,RE, and Be are added into AZ91D alloy, such that the ignition point ofthe material is increased to 710° C. The patent application No.CN201410251364.7 discloses a flame-retardant and high-strength magnesiumalloy and its preparation method, wherein the alloy has a composition ofMg—Al—Y—CaO, a flame retardant temperature≥745° C., and aroom-temperature tensile strength≥231 MPa. These alloys involved in theabove patents have a good flame retardant property, but poor mechanicalproperties, which limits its application and development.

The corrosion resistance of the alloy has a crucial influence on itsapplication.

The corrosion resistance of the current commercial magnesium alloys ispoor, and the corrosion rate of AZ31 magnesium alloy is about 4.5mg·cm⁻²·d⁻¹. In the patent application No. CN201010120418.8, thecorrosion rate thereof can be reduced to as low as 0.98 mg·cm⁻²·d⁻¹ byadding Y-rich mischmetal into AZ31. The corrosion rate of AZ91 alloy isabout 1.58 mg·cm⁻²·d⁻¹. The patent application No. CN200910248685.0discloses a magnesium alloy with corrosion resistance, wherein thecorrosion rate thereof is remarkably reduced to as low as 0.64mg·cm⁻²·d⁻¹ by adding a certain amount of Cd into AZ91. The patentapplication No. CN201410521001.0 discloses a magnesium alloy withcorrosion resistance, wherein the corrosion rate thereof can reach to aslow as 0.54 mg·cm⁻²·d⁻¹ by adding V element into AZ91. The corrosionrate of the rare-earth containing magnesium alloy is lower, and thecorrosion rate of WE43 alloy is about 0.6 mg·cm⁻²·d⁻¹. The patentapplication No. CN200910099330.X discloses a Mg—Nd—Gd—Zn—Zr alloy withCaO added therein, wherein the corrosion rate thereof can be as low as0.16 mg·cm⁻²·d⁻¹, but after T6 treatment, the strength thereof is poor,and the high cost also limits its application and development.

Furthermore, the fracture toughness of the magnesium alloy are generallylow, so it is of important significance to develop a magnesium alloywith high fracture toughness for improving the security and reliabilityduring the service of the magnesium alloys.

As a metallic structural material having a wide application prospect,the magnesium alloy still faces a lot of technical problems urgent to besolved in the practical application. In order to promote the applicationof the magnesium alloy, there is a need to develop alloys with goodoverall performances while ensuring the acceptable material cost.

Summary of Invention

In order to overcome the problem of overhigh rare earth content andinsufficient overall performance of the current high-strength magnesiumalloy, the present invention develops Mg—Gd—Y—Zn—Zr alloys with lowrare-earth content and high strength, high toughness, and excellentanti-flammability and corrosion resistance, and a process for preparingthe same. The total content of rare earth is not more than 11 wt. %, andthe process is simple, the operation is easy, and the cost is low, sothat the problem of complicated preparation process and high preparationcost for the alloy is overcome.

The object of the present invention is achieved by the technicalsolutions as follows.

A Mg—Gd—Y—Zn—Zr alloy with high strength and toughness, corrosionresistance and anti-flammability, wherein the components and the masspercentages thereof in the alloy are: 3.0%≤Gd≤9.0%, 1.0%≤Y≤6.0%,Gd+Y≤11.0%, 0.5%≤Zn≤3.0%, 0.2%≤Zr≤1.5%, and the balance being Mg andinevitable impurities.

A process for preparing the aforementioned Mg—Gd—Y—Zn—Zr alloy with highstrength and toughness, corrosion resistance and anti-flammability,specifically carried out by steps of:

(1) calculating and burdening according to the alloy composition,wherein the raw materials Gd, Y and Zr are added in the form of masteralloys of Mg-30 wt. % Gd, Mg-30 wt. % Y and Mg-30 wt. % Zr,respectively, and Mg and Zn are added in the form of industrially pureMg and pure Zn, respectively;

(2) increasing the temperature of a smelting furnace to 760-850° C.,adding the pure Mg and pure Zn prepared in step 1 into the smeltingfurnace under the protection of mixed gases of CO₂+10 vol % SF₆;

(3) reducing the temperature of the furnace to 730-780° C. after thepure Mg and pure Zn added in step (2) are completely melted, adding theMg—Gd master alloy, the Mg-Y master alloy, and the Mg—Zr master alloy inthis order, to obtain a melt;

(4) adjusting the temperature of the furnace to 700-750° C., removingthe slag on the surface of the melt, refining the melt for 10-20 minutesby introducing preheated argon at the bottom of the furnace, to improvethe purity of the melt;

(5) increasing the temperature to 730-760° C., transferring the meltinto a holding furnace under the pressure of 0.01 to 0.02 MPa, andholding for 1-3 hours; and

(6) reducing the temperature to 700-720° C., casting the melt preparedin step (5) at a rate of 42 mm/min, cooling and crystalizing the castingot with cooling water at room temperature and a pressure of 0.02 MPa,to finally obtain a large ingot of the Mg—Gd—Y—Zn—Zr alloy with adiameter of 170 mm and a length≥2.5 m by casting;

(7) conducting a homogenization treatment on the ingot at a temperatureof 450-550° C. for 8-24 hours, and then quenching in warm water at50-80° C.;

(8) conducting an indirect extrusion on the ingot after thehomogenization treatment, wherein the extrusion temperature iscontrolled at 350-450° C., the extrusion ratio is 8-20, and the rainspeed is 0.05-5 mm/s; and

(9) conducting an isothermal aging treatment on the extruded alloy at175-225° C. for a holding time of 0.5-200 hours, quenching and coolingthe sample in warm water at 50-80° C. after the aging treatment, toobtain the target alloy.

The present invention has the following beneficial effects.

1. The present invention can produce a magnesium alloy with highstrength and toughness and low rare earth content by employingconventional preparation processes. The extrusion process is simple andeasy to operate, and has a wide application range.

2. The Mg—Gd—Y—Zn—Zr alloy not only has excellently high strength andtoughness, but also has excellent corrosion resistance and flameretardant property. As compared with the commonly used commercialmagnesium alloys such as AZ91, ZK60 and WE43, the overall performancethereof has a significant improvement.

3. When the total amount of rare earth in the Mg—Gd—Y—Zn—Zr alloys is7-11 wt %, the alloy has a tensile strength≥428 MPa, a yieldstrength≥409 MPa, an elongation≥10.1%, a fracture toughness (Kqvalue)≥21.3 MPa·m^(1/2), a corrosion rate in the salt spray test (3.5%NaCl)≤0.56 mg·cm⁻²·d⁻¹, and an ignition point≥708° C.

4. Both the fracture toughness and the corrosion resistance of theMg—Gd—Y—Zn—Zr alloy are better than those of WE43 alloy, while the flameretardant property thereof is equivalent to that of WE43 alloy.

DETAILED DESCRIPTION OF THE INVENTION

The technical solution of the present invention will be furtherdescribed below by referring to the Examples. However, the presentinvention is not limited thereto, and any modifications or equivalentalternatives of the technical solution of the present invention, withoutdeparting from the spirit and scope of the technical solution of thepresent invention, should be included in the scope of the presentinvention.

EXAMPLE 1

In the Example, the components and the mass percentages thereofcontained in the Mg—Gd—Y—Zn—Zr alloy with high strength are: Gd 8.0%, Y3.0%, Zn 1.0%, Zr 0.5%, and the balance being Mg and inevitable impurityelements. The specific preparation method for the alloy is carried outaccording to the following steps:

1. weighing pure Mg, pure Zn, Mg—Y master alloy, Mg—Gd master alloy andMg—Zr master alloy according to the ratio of 8% Gd, 3% Y, 1% Zn, 0.5% Zrand the balance of Mg based on mass percentage;

2. heating the smelting furnace to 800° C., adding the pure Mg and pureZn prepared in step 1 into the smelting furnace under the protection ofmixed gases of CO₂+10 vol % SF₆;

3. reducing the temperature of the furnace to 760° C. after the pure Mgand pure Zn are completely melted, adding the Mg—Gd master alloy, theMg—Y master alloy, and the Mg—Zr master alloy in this order, to obtain amelt;

4. reducing the furnace temperature to 740° C., removing the slag on thesurface of the melt, refining the melt for 15 minutes by introducingpreheated argon at the bottom of the furnace, to improve the purity ofthe melt;

5. increasing the temperature to 750° C., transferring the melt into aholding furnace under a pressure of 0.02 MPa, and holding for 2 hours,

6. reducing the temperature to 705° C., casting the melt prepared instep 5 at a rate of 42 mm/min, cooling and crystalizing the cast ingotwith cooling water at room temperature and a pressure of 0.02 MPa, tofinally obtain a large ingot of the Mg—Gd—Y—Zn—Zr alloy with a diameterof 170 mm and a length of 2.75 m by casting;

7. conducting a homogenization treatment on the ingot at 500° C. for 12hours, then quenching in warm water at 80° C.;

8. conducting an indirect extrusion on the ingot after thehomogenization treatment, wherein the extrusion temperature iscontrolled at 390° C., the extrusion ratio is 12:1, and the rain speedis 0.1 mm/s; and

9. conducting an isothermal aging treatment on the extruded alloy at200° C. for 72 hours, and quenching the sample in warm water at 80° C.after the aging treatment, to obtain the target alloy.

The resultant alloy of the Example has a tensile strength of 465 MPa, ayield strength of 437 MPa, and an elongation of 10.8%. See Table 1 fordetails.

EXAMPLE 2

In the Example, the components and the mass percentages thereofcontained in the Mg—Gd—Y—Zn—Zr alloy with high strength are: Gd 8.4%, Y2.4%, Zn 0.6%, Zr 0.4%, and the balance being Mg and inevitable impurityelements. The preparation method of the Mg—Gd—Y—Zn—Zr alloy with highstrength is: firstly, weighing pure Mg, pure Zn, Mg—Y master alloy,Mg—Gd master alloy and Mg—Zr master alloy according to the ratio of 8.4%Gd, 2.4% Y, 0.6% Zn, 0.4% Zr and the balance of Mg based on masspercentage; casting the alloy according to steps 2-6 in Example 1;conducting the homogenization treatment on the ingot at 500° C. for 12hours, then quenching in the warm water at about 80° C.; conducting theindirect extrusion on the ingot after the homogenization treatment,wherein the extrusion temperature is controlled at 400° C., theextrusion ratio is 12:1, and the rain speed is 0.1 mm/s; conducting theisothermal aging treatment on the extruded alloy at 200° C. for 118hours, and quenching the sample in the warm water at 80° C. after theaging treatment, to obtain the target alloy. The properties of the alloyare shown in Table 1.

EXAMPLE 3

In the Example, the components and the mass percentages thereofcontained in the Mg—Gd—Y—Zn—Zr alloy with high strength are: Gd 6.7%, Y1.3%, Zn 0.6%, Zr: 0.5%, and the balance being Mg and inevitableimpurity elements. The preparation method of the Mg—Gd—Y—Zn—Zr alloywith high strength is: firstly, weighing pure Mg, pure Zn, Mg—Y masteralloy, Mg—Gd master alloy and Mg—Zr master alloy according to the ratioof 6.7% Gd, 1.3% Y, 0.6% Zn, 0.5% Zr and the balance of Mg based on masspercentage; casting the alloy according to steps 2-6 in Example 1;conducting the homogenization treatment on the ingot at 510° C. for 8hours, then quenching in the warm water at about 80° C.; conducting theindirect extrusion on the ingot after the homogenization treatment,wherein the extrusion temperature is controlled at 400° C., theextrusion ratio is 12:1, and the ram speed is 0.1 mm/s; conducting theisothermal aging treatment on the extruded alloy at 200° C. for 84hours, and quenching the sample in the warm water at 80° C. after theaging treatment, to obtain the target alloy. The properties of the alloyare shown in Table 1.

EXAMPLE 4

In the Example, the components and the mass percentages thereofcontained in the Mg—Gd—Y—Zn—Zr alloy with high strength are: Gd 8.4%, Y0.8%, Zn 0.7%, Zr 0.6%, and the balance being Mg and inevitable impurityelements. The preparation method of the Mg—Gd—Y—Zn—Zr alloy with highstrength is: firstly, weighing pure Mg, pure Zn, Mg—Y master alloy,Mg—Gd master alloy and Mg—Zr master alloy according to the ratio of 8.4%Gd, 0.8% Y, 0.7% Zn, 0.6% Zr and the balance of Mg based on masspercentage; casting the alloy according to steps 2-6 in Example 1;conducting the homogenization treatment on the ingot at 510° C. for 8hours, then quenching in the warm water at about 80° C.; conducting theindirect extrusion on the ingot after the homogenization treatment,wherein the extrusion temperature is controlled at 400° C., theextrusion ratio is 12:1, and the ram speed is 0.1 mm/s; conducting theisothermal aging treatment on the extruded alloy at 200° C. for 84hours, and quenching the sample in the warm water at 80° C. after theaging treatment, to obtain the target alloy. The properties of the alloyare shown in Table 1.

EXAMPLE 5

In the Example, the components and the mass percentages thereofcontained in the Mg—Gd—Y—Zn—Zr alloy with high strength are: Gd 7.1%, Y2.0%, Zn 1.1%, Zr 0.5%, and the balance being Mg and inevitable impurityelements. The preparation method of the Mg—Gd—Y—Zn—Zr alloy with highstrength is: firstly, weighing pure Mg, pure Zn, Mg—Y master alloy,Mg—Gd master alloy and Mg—Zr master alloy according to the ratio of 7.1%Gd, 2.0% Y, 1.1% Zn, 0.5% Zr and the balance of Mg based on masspercentage; casting the alloy according to steps 2-6 in Example 1;conducting the homogenization treatment on the ingot at 510° C. for 8hours, then quenching in the warm water at about 80° C.; conducting theindirect extrusion on the ingot after the homogenization treatment,wherein the extrusion temperature is controlled at 400° C., theextrusion ratio is 12:1, and the ram speed is 0.1 mm/s; conducting theisothermal aging treatment on the extruded alloy at 200° C. for 84hours, and quenching the sample in the warm water at 80° C. after theaging treatment, to obtain the target alloy. The properties of the alloyare shown in Table 1.

TABLE 1 The properties of the alloys in the Examples and WE43(comparative sample) Weight loss in salt Kq spray test Ignition UTS YS ε(MPa- mg · point (MPa) (MPa) (%) m^(1/2)) (cm⁻² · d⁻¹) (° C.) Example 1465 437 10.8 31.2 0.32 748 Example 2 455 425 10.2 25.1 0.37 722 Example3 428 409 10.1 21.6 0.50 728 Example 4 436 422 11.3 21.3 0.56 746Example 5 451 423 10.7 22.4 0.37 708 WE43 352 243 11.5 15.0 0.61 765

The present invention obtains a wrought magnesium alloy having superioroverall performances with a small amount of rare earth element byadjusting the proportion of the alloy elements and by conventionalcasting, extrusion and heat treatment processes. At room-temperature,the tensile strength thereof is 428-465 MPa, the yield strength is409-437 MPa, and the elongation is 10.1%-14.4%; meanwhile, it also hasexcellent fracture toughness, corrosion resistance and flame retardantproperty. The cost of the alloy is reduced while the strength of thealloy is maintained.

1. A Mg—Gd—Y—Zn—Zr alloy wherein the components and the mass percentagesthereof in the Mg—Gd—Y—Zn—Zr alloy comprise from 3.0% to 9.0% Gd, from0.8% to 6.0% Y, from 0.5% to 3.0% Zn, from 0.2% to 1.5% Zr, the balancebeing Mg and inevitable impurities.
 2. The Mg—Gd—Y—Zn—Zr alloy accordingto claim 1, wherein Gd+Y is 11.0% or less.
 3. The Mg—Gd—Y—Zn—Zr alloyaccording to claim 1, the alloy comprising 8.0% Gd, 3.0% Y, 1.0% Zn,0.5% Zr, the balance being Mg and inevitable impurities.
 4. TheMg—Gd—Y—Zn—Zr alloy according to claim 1, the alloy comprising Gd: 8.4%,Y: 2.4%, Zn: 0.6%, Zr: 0.4%, the balance being Mg and inevitableimpurities.
 5. The Mg—Gd—Y—Zn—Zr alloy according to claim 1, the alloycomprising Gd: 6.7%, Y: 1.3%, Zn: 0.6%, Zr: 0.5%, the balance being Mgand inevitable impurities.
 6. The Mg—Gd—Y—Zn—Zr alloy according to claim1, the alloy comprising: Gd: 8.4%, Y: 1%, Zn: 0.7%, Zr: 0.6%, thebalance being Mg and inevitable impurities.
 7. The Mg—Gd—Y—Zn—Zr alloyaccording to claim 1, the alloy comprising Gd: 7.1%, Y: 2.0%, Zn: 1.1%,Zr: 0.5%, the balance being Mg and inevitable impurities.
 8. A processfor preparing the Mg—Gd—Y—Zn—Zr alloy according to any of claims 1 to 7,characterized in that the process comprises: (1) calculating andburdening according to the alloy, wherein Gd, Y and Zr are added in theform of master alloys of Mg-30 wt. % Gd, Mg-30 wt. % Y and Mg-30 wt. %Zr, respectively, and Mg and Zn are added in the form of industriallypure Mg and pure Zn, respectively; (2) increasing a first temperature ofa smelting furnace to from 760 to 850° C., adding industrially pure Mgand pure Zn prepared in step (1) into the smelting furnace underprotection of mixed gases of CO₂+10vol % SF₆; (3) reducing the firsttemperature to a second temperature of the smelting furnace to from 730to 780° C. after the industrially pure Mg and industrially pure Zn addedin step (2) are completely melted, adding the Mg—Gd master alloy, theMg—Y master alloy, and the Mg—Zr master alloy in this order, to obtain amelt; (4) adjusting the second temperature to a third temperature of thesmelting furnace to from 700 to 750° C., removing slag on a surface ofthe melt, refining the melt for from 10 to 20 minutes by introducingpreheated argon at a bottom of the smelting furnace, to improve thepurity of the melt; (5) increasing the third temperature to a fourthtemperature to from 730 to 760° C., transferring the melt into a holdingfurnace under the pressure of from 0.01 to 0.02 MPa, and holding forfrom 1 to 3 hours; and (6) reducing the fourth temperature to a fifthtemperature to from 700 to 720° C., casting the melt prepared in step(5), cooling and crystalizing a cast ingot with cooling water at roomtemperature, to finally obtain a large ingot of the Mg—Gd—Y—Zn—Zr alloywith a diameter of 170 mm and a length 2.5 m or more by casting.
 9. Theprocess for preparing the Mg—Gd—Y—Zn—Zr alloy according to claim 8,wherein casting the melt prepared in step (5) is performed at a thecasting rate is 42 mm/min, and cooling and crystalizing the cast ingotwith cooling water is performed at a pressure of the cooling water is0.02 MPa.
 10. The process for preparing the Mg—Gd—Y—Zn—Zr alloyaccording to claim 8, wherein the process further comprises stops of:(7) conducting a homogenization treatment on the large ingot of theMg—Gd—Y—Zn—Zr alloy at a temperature of from 450 to 550° C. for from 8to 24 hours, and then quenching in warm water at a temperature from 50to 80° C.; (8) conducting an indirect extrusion on the large ingot afterthe homogenization treatment, wherein the extrusion temperature iscontrolled at a temperature from 350 to 450° C., an extrusion ratio isfrom 8 to 20, and a ram speed is 0.05-5 mm/s; and (9) conducting anisothermal aging treatment on the extruded alloy at from 175 to 225° C.for a holding time of from 0.5 to 200 hours, quenching and cooling thesample in warm water at 50-80° C. after the isothermal aging treatment,to obtain the Mg—Gd—Y—Zn—Zr alloy.